DSL is a high-speed transmission technology that transmits data through a telephone twisted pair, namely, an Unshielded Twist Pair (UTP). The twisted pairs used by the DSL are unshielded, and electromagnetic interference between line pairs generates bit errors, so transmission quality of the line is deteriorated.
In an existing DSL system, an error-correcting code is generally used to correct errors generated in the sent data. For example, Reed-Solomon (RS) codes are used to mitigate the impact caused by the impulse noise by virtue of the code gain provided additionally by the RS codes, so as to increase the stability of the DSL system. However, with the increase of the DSL bandwidth and the increase of the rate, when an original RS code is used, a RS codeword with more check bytes and shorter length is needed. In this case, a net gain of the RS code changes to a negative value, which is adverse to the increase of the rate.
When the system is required to provide high impulse noise protection, and a small delay is required, the RS code will introduce more redundancy (namely, check bytes), which makes the net gain of the code be negative and make the rate decrease. In more circumstances, the Impulse Noise Protection (INP) provided by the system is unable to tackle the bit errors of data caused by the impulse noise in the line. When a set noise margin is small, the RS coding is mainly designed to tackle the impact (namely, bit errors) caused by a stable noise onto the DSL system. In this case, the capability of the system in resisting the impulse noise is further weakened.
In order to better tackle the impact caused by the impulse noise onto the DSL system, a physical-layer retransmission scheme emerges accordingly, especially a retransmission mechanism at a Physical Media Specific Transmission Convergence (PMS-TC) sublayer. The retransmission mechanism is capable of retransmitting the data damaged by the impulse noise, so as to reduce a bit error rate of the line and improve the service stability. This technology alleviates the impact caused by the impulse noise on the system to some extent and improves the service stability of the system at the cost of increasing a service delay and reducing a net rate of the line. In the existing DSL system, the PMS-TC layer is incapable of acquiring a service type of the data at the PMS-TC layer, and the same path may bear multiple services at the same time. An existing retransmission system at the PMS-TC layer additionally increases a delay by more than 10 ms. For services (such as a video service) having high requirements for the bit error rate but low requirements for the delay, the retransmission technology reduces the bit error rate of the line and improves Quality of Experience (QoE) of a user in a range of a tolerable delay. However, some services (such as VoIP) having low requirements for the bit error rate but high requirements for the delay still exist in a network, and the delay increased by retransmission for such services is intolerable to the user, which greatly affects the QoE of the user, and even may causes complaints of the user.